A. Field of the Invention
The present invention relates to the field of ophthalmic instruments and methods, and more particularly to an improved method of non-contact tonometry for measuring intra-ocular pressure (IOP) of an eye.
B. Description of the Prior Art
Tonometers for measuring IOP were originally developed as xe2x80x9ccontactxe2x80x9d type instruments, meaning that a portion of the instrument is brought into contact with the cornea during the measurement procedure. A well-known instrument of this type is the Goldmann applanation tonometer (GAT) originally developed during the 1950s. The GAT measures the force required to flatten (xe2x80x9capplanatexe2x80x9d) a known area of the cornea, and is used today as a standard against which other types of tonometers are compared to assess measurement accuracy.
Patient discomfort caused by contact tonometers such as the GAT led to the development of xe2x80x9cnon-contactxe2x80x9d tonometers (NCTs) which operate by directing an air pulse generated by a pump mechanism through a discharge tube aimed at the cornea to cause applanation. As the cornea is deformed by the fluid pulse, an opto-electronic system monitors the cornea by detecting corneally reflected light from a beam obliquely incident upon the cornea, and a peak detector signal occurs at the moment of applanation when the reflecting surface of the cornea is flat.
In state of the art NCTs, a pressure transducer measures the pump plenum pressure as the pulse is generated to provide a plenum pressure signal, which typically approximates a Gaussian distribution, whereby the plenum pressure at the moment applanation is achieved can be determined. The plenum pressure at applanation is then converted to an IOP value in units of mmHg using a linear regression equation stored during instrument clinical calibration relative to GAT as a reference. A primary index of an NCT""s reliability is the standard deviation of differences Sd of matched pairs of NCT and GAT clinical readings.
While NCTs provide reasonably reliable IOP measurements, recent studies indicate that corneal thickness effects can have a significant impact on NCT readings. IOP readings are falsely inflated because the air pulse expends some of its energy xe2x80x9cbendingxe2x80x9d the corneal tissue itself, as opposed to displacing intra-ocular fluid pressing on the cornea. See, for example, Copt R-P, Thomas R, Mermoud A, Corneal Thickness in Ocular Hypertension, Primary Open-angle Glaucoma, and Normal Tension Glaucoma, Arch Ophthalmol. Vol. 117:14-16 (1999); Emara B, Probst L E, Tingey D P, Kennedy D W, et al., Correlation of Intraocular Pressure and Central Corneal Thickness in Normal Myopic Eyes After Laser in situ Keratomileusis, J Cataract Refract Surg, Vol. 24:1320-25 (1998); Stodtmeister R, Applanation Tonometry and Correction According to Corneal Thickness, Acta Ophthalmol Scand, Vol. 76:319-24 (1998); and Argus W A, Ocular Hypertension and Central Corneal Thickness, Ophthalmol, Vol. 102:1810-12 (1995). For persons with relatively thick corneas, IOP values measured under prior art methodology can be significantly effected. Heretofore, attempts to correct measured IOP for corneal thickness have typically involved measuring corneal thickness by additional instrument means and correcting measured IOP by an amount based upon the measured corneal thickness. U.S. Pat. No. 5,4740,66 (Grolman) issued Dec. 12, 1995 ascribes to this approach.
During a non-contact IOP measurement, the cornea is actually deformed from its original convex state through a first state of applanation to a slightly concave state, and is allowed to return from concavity through a second state of applanation to convexity as the air pulse decays. Indeed, a second peak is known to occur in the applanation signal corresponding to the second state of applanation. Heretofore, non-contact tonometry methods have only taken into account plenum pressure associated with the first state of applanation for inputting to a regression equation to calculate IOP, and have ignored plenum pressure associated with the second state of applanation.
It is therefore an object of the present invention to provide an improved non-contact tonometry method that provides a more accurate measurement of true IOP.
It is another object of the present invention to provide a non-contact tonometry method that eliminates corneal thickness as a factor effecting IOP measurement values.
It is a further object of the present invention to achieve the aforementioned objects using existing NCT hardware.
In view of these and other objects, a non-contact tonometry method according to a first embodiment of the present invention comprises the following steps. Initially, as in the prior art, a fluid pulse is directed at the cornea to cause reversible deformation of the cornea from convexity, through first applanation, to concavity, and back through second applanation to convexity. Corneal deformation is monitored as a function of time to generate a signal indicating times t1 and t2 of first applanation and second applanation, respectively. As corneal deformation is monitored, so to is a plenum pressure of the pump mechanism generating the fluid pulse in order to determine a first plenum pressure PP1 and a second plenum pressure PP2 corresponding to first and second applanation times t1 and t2, respectively. First plenum pressure PP1 is input as a coefficient of a first regression equation to yield a first intra-ocular pressure value IOP1, and second plenum pressure PP2 is input as a coefficient of a second regression equation to yield a second intra-ocular pressure value IOP2. The two values IOP1 and IOP2 are then averaged to compute a final reported intra-ocular pressure value IOPf. Alternatively, an average PPAVG of the first and second plenum pressures could be input as a coefficient of a single corresponding regression equation to compute IOPf.